This question seems like a "community wiki" question, but I don't see how to change it myself. If you have permission, please do. Also, I know I could have just inverted the first image, but that seemed like cheating. :)
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Jeremy SteinAug 10 '11 at 15:03

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I don't think that this should be CW. However, I do think it's potentially too localised as it only concerns sockets in the US.
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ChrisF♦Aug 10 '11 at 15:22

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I think this is possible to answer this question, and that Niall covered all the bases... ;)
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Alex FeinmanAug 10 '11 at 15:43

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@Chris, It is definitely not too localized. Localization (in this sense of the word) has nothing to do with location of the problem. (There is a podcast that discusses this in great length.)
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jjnguyAug 10 '11 at 18:51

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The first one looks right because it's a face. =o An upside down face can't be to code!
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RayAug 11 '11 at 1:55

9 Answers
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The ground pin is usually facing down, and, if I recall correctly, when I installed some tamper-proof receptacles recently, the printing on them was right-side-up with the ground pin down. I believe it's also a requirement in some localities.

On the other hand, people supposedly install the ground pin facing up as a safety precaution: if you're falling or dropping something onto a plug that's not securely plugged in, you'll hit the ground pin first so there's no risk of a shock. If you've got both orientations within any one room, it's possible the installer didn't know what he was doing.

If your locality doesn't have a requirement for them to be facing down, I would just pick the orientation that you prefer and re-install the ones that are in the other orientation to match.

I once lived in a small town where nearly all the light switches were upside down (resulting in "NO" rather than "ON'). Must have been one confused electrician.
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BryceJun 19 '12 at 2:40

In my house all of the outlets have ground pin down except for the switched outlets, those were installed "upside down", apparently to indicate that they are switched. When I moved in, I couldn't figure out why a few outlets were upside down.... until I plugged in a lamp and figured it out.
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JohnnyFeb 6 at 21:19

It's a preference question (barring and local code requirement). Where I was able to, I installed my outlets horizontally to avoid issues where one extension/power cord was above another.
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BrownRedHawkFeb 6 at 21:45

There are many transformers (wall warts), chargers, etc, that are polarized so the bulk of them is hanging down when the ground pin is down. If the outlet is installed with ground facing up, then these polarized wall warts will be facing up and often that will cause them to sit on the wall at an angle (since the center of gravity is higher than the plugs), exposing the prongs and creating a potential safety hazard.

You Americans should get proper plugs and sockets like the ones we have in the UK.
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ChrisF♦Aug 10 '11 at 15:48

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@ChrisF: Touche. I'm also of the opinion that we should adopt the metric system. But then again, I'm an engineer and hate dealing with ridiculous units like BTUs and horsepower.
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DoresoomAug 10 '11 at 16:37

Wall warts don't have to be polarized, and I can't remember the last one I saw that was; the diode/capacitor system that converts AC to DC ensures that one side of the DC will always be + and the other will always be -. As long as the circuit is continuous from one blade to the other, the diode will only let current flow in one direction. It would only matter which blade was hot if the adapter had a single-pole switch (in which case the switch has to disconnect the "hot" side).
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KeithSAug 12 '11 at 22:01

An interesting idea, but I'd rather use a recep of a different color to indicate it was switched.
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Jay BazuziAug 11 '11 at 4:58

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The switched outlets in our house are split -- top half switched, bottom half always on. We just have to remember which ones they are!
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TomGAug 13 '11 at 2:32

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@TomG: same in our house - most rooms are split that way. It is incredibly annoying, and I'm considering rewiring everything.
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AlgerAug 13 '11 at 2:56

You can replace the split switched receps with single receps, so they don't have to be split.
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Jay BazuziApr 6 '12 at 5:35

Folks a split receptacle is just a normal one with a connecting pin taken off. Just put in a regular receptacle and connect it to the always hot or switched leg depending on your preferences.
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Tim Post♦Aug 26 '12 at 15:18

Ground pin down

In most situations the cord would be hanging down, which could cause the plug to tend to pull out in a downward direction. As a receptacle ages the contacts may lose a bit of their grip, allowing the plug to become dislodged. If the ground pin is down, it will be the last to lose contact with the with the receptacles contacts. If the ground pin is up, it could lose contact before the ungrounded blade leading to a potentially bad situation.

The ground pin being thicker and sturdier, can prevent the plug from becoming dislodged when holding the weight of a cord.

Ground pin up

If the plug was slightly dislodged, metal objects falling along the wall would short across the hot and neutral blades.

If the junction box were to fill with water, the water would reach the hot/neutral first. Which would short the receptacle and trip the breaker (Sounds odd, but I've actually heard this argument).

Conclusion

While both sides of the argument passionately believe they are right, there are no "official" standards on which orientation is correct. In my opinion, I follow the "make first, break last" principal. So I prefer ground pin down.

That's a really good question. I have always considered "ground-pin-down" to be "right-side-up"; the plug looks like a face.

However, there are plenty of arguments to go either way; having the ground pin up is safer in terms of falling against a grounded plug, while ground-pin-down means that "wall warts" that have polarized blades or a ground pin can hang with their bulk positioned under the switch. Many AC adapters, though, have now gone toward either the "brick" style (with a short cord run on both ends of the adapter) or the "one-spot" (compact, and oriented either "outward" or "sideways"; either way the plug fits better onto a standard power strip).

Looking at Lowe's' catalog of 3-prong electrical outlets, it's a crap shoot as to which way each plug has been photographed. Whatever you prefer, plugs go into J-boxes either way, so you can do it however you like as the situation may call for.

I think the real answer varies not only by your local codes, but also if the work is commercial or residential.

I was always told that it is code (NEC) for commercial applications to have the ground on the top for the safety reasons listed in other answers. I almost always see them up at work and other places, so assume that is true. For residential, I dont think it is specified so it is up to the electrician's preference. In many cases, electricians that do a lot or commercial work, just keep it the same for residential out of habit.

The explanation I was told was that if a metal face plate is used, the ground pin should be at the top. This way, if the plate were to become loose with a plug that is not all the way pushed in, the plate would contact the ground pin first as opposed to the hot.

Traditionally, the ground pin is at the bottom. My grandfather installed them ground pin up because he believed a young child might otherwise perceive a "face" and decide to play with the outlet. I think, if you have a young child, just go ahead and get some outlet guards.